Electrophysiological activity from over the cerebellum and cerebrum during eye blink conditioning in human subjects

We report the results of an experiment in which electrophysiological activity was recorded from the human cerebellum and cerebrum in a sample of 14 healthy subjects before, during and after a classical eye blink conditioning procedure with an auditory tone as conditional stimulus and a maxillary nerve unconditional stimulus. The primary aim was to show changes in the cerebellum and cerebrum correlated with behavioral ocular responses. Electrodes recorded EMG and EOG at peri-ocular sites, EEG from over the frontal eye-fields and the electrocerebellogram (ECeG) from over the posterior fossa. Of the 14 subjects half strongly conditioned while the other half were resistant. We confirmed that conditionability was linked under our conditions to the personality dimension of extraversion-introversion. Inhibition of cerebellar activity was shown prior to the conditioned response, as predicted by Albus (1971). However, pausing in high frequency ECeG and the appearance of a contingent negative variation (CNV) in both central leads occurred in all subjects. These led us to conclude that while conditioned cerebellar pausing may be necessary, it is not sufficient alone to produce overt behavioral conditioning, implying the existence of another central mechanism. The outcomes of this experiment indicate the potential value of the noninvasive electrophysiology of the cerebellum

[In Press] Effects of stimulus intensity and frequency on the force and timing of sensorimotor synchronisation

We report an experiment to investigate possible vestibular effects on finger tapping to an auditory anapaest rhythm. In a sample of 10 subjects, index finger acceleration and tapping force were recorded along with extensor/flexor activity and the associated electroencephalographic activity measured at central and cerebellar surface electrodes. In a prior session with a standard short airconducted 500-Hz pip, vestibular evoked myogenic potential thresholds were measured and subsequently used to set the acoustic intensity. During the main experiment subjects were asked to synchronise tapping to the pips arranged in the anapaest at two different frequencies, 500 Hz vs 5 kHz, so that only the low-frequency high-intensity condition was a vestibular, as well as an auditory stimulus. We hypothesised that a vestibular effect would manifest in an interaction between the frequency and intensity factors for a range of dependent measures of tapping performance. No clear evidence was found for vestibular effects, but this was likely due to the confounding effects of an independent effect of intensity and the relative weakness of the acoustic vestibular stimulus. However, the data did show novel evidence for two distinct timing processes for the flexion and extension stages of a tap cycle and two distinct timing strategies, which we refer to as ‘staccato’ and ‘legato’, characterised by different profiles of force and extension

Physiological properties and clinical characteristics of vestibulo-collic and vestibulo-ocular reflexes in humans

Vestibular evoked myogenic potentials (VEMPs) are short-latency reflexes recorded from the cervical (cVEMPs) and ocular (oVEMPs) muscles. Both reflexes are commonly elicited using air-conducted (AC) and bone-conducted (BC) stimulus modalities. Despite their growing clinical prevalence, the properties they exhibit remain less well studied. Mapping of the oVEMP was carried out for AC and BC modalities. A stable latency and larger more linear response to gaze angle were found for a slightly lateral recording electrode compared to the conventional one. Gaze angle can potentially differentiate between extraocular sources depending on the electrode montage used. AC frequency tuning of cVEMPs and oVEMPs were found to be broadly similar even when middle ear attenuation effects were accounted for. Stimulus phase did not modulate the observed frequency effects. AC stimuli tuning patterns and phase effects differ from those of BC stimuli which suggest differences in their mechanism of activating vestibular receptors. Both BC 500 Hz and BC impulses are effective means of evoking cVEMPs and oVEMPs from mid sagittal skull sites. The effects depend upon both location and phase and differ for cVEMPs and oVEMPs with the most consistent effects occurring for AFz and Fz stimulation. Dissociation between stimulus modalities was found for stimulus phase. Phase did not alter the properties of AC reflexes whereas BC reflexes were affected, more strongly for oVEMPs than for cVEMPs. Differences in phase effects between modalities may relate to their differing mechanisms of transduction. In vestibular neuritis, sparing of cVEMPs and abnormal oVEMPs were observed for AC and BC 500 Hz stimuli while BC impulses were generally affected for both reflexes. Modelling of the findings to AC and BC 500 Hz demonstrated strong utricular dependence for oVEMPs and strong saccular dependence for cVEMPs. Utricular contributions were demonstrated for both reflexes using BC impulses. Properties of cVEMPs and oVEMPs in superior canal dehiscence were not equally useful in separating patients from controls. Thresholds (AC 500 Hz cVEMPs and BC 500 Hz oVEMPs) and amplitudes (AC 500 Hz cVEMPs and oVEMPs) showed high sensitivity and specificity. Amplitudes of cVEMPs to AC and BC 500 Hz were shown to saturate whereas oVEMP amplitudes did not